1. Field of the Invention
This invention relates to bending plates of thermoplastic material, in particular glass, as they pass on a conveyor in approximately horizontal position, by raising the plates from the conveyor with heated gas currents and applying them against a bending form placed above.
2. Description of Related Art
It is known from European patent document EP 0 005 306 how to bend a glass plate by lifting it to apply it against a carrying surface by means of a plurality of independent hot air jets directed toward the lower face of the glass plate, the jets being produced by orifices in the form of nozzles in the upper wall of a compressed air vessel, and then dropping the plate from this carrying surface onto a bending mold. The plate bends by the sole effect of its own weight being dropped against the mold. Optionally the plate can also be pressed between the upper carrying surface and the mold.
The carrying surface is, moreover, pierced with orifices through which the glass plate is held by suction at the same time as it is lifted by the air jets exerted against its lower face.
The bending mold may consist of a frame, also called a skeleton, which is intended to bear against the marginal area at the periphery of the glass plate to be bent.
In this known technique, the bending skeleton exerts a mechanical action on the periphery of the glass plate, which, for the bending operation, has been brought to a high temperature in order to be deformable.
In the most favorable case, this action results only from the weight itself of the glass plate but, on the other hand, when the glass plate is forcefully projected in the direction of the skeleton or when it is pressed between the carrying surface and the skeleton, the mechanical action is much greater. This mechanical action on the periphery of the glass plate is harmful; it is characterized by faults in the appearance of the glass surface and by warping that can alter its optical properties.
Such defects in appearance or geometry in the peripheral marginal zone of the glass plates are particularly bothersome when automobile windows are involved. In automobile windows, optical quality is fundamental to safety. Further, modern installation techniques may put the windows directly in contact with the automobile body, without insertion of a gasket and often by simple gluing. Thus the entire surface of the windows, right down to its very edge, can be seen from the outside, which makes a defect in appearance, even in the peripheral marginal zone, intolerable.
Further, when the glass plate is placed or projected against the skeleton, it is not possible to prevent undesirable uncontrolled formation, due to gravity alone of the central zones of the plate, which are not supported by the skeleton.